A large majority of college marijuana users engage in simultaneous alcohol and marijuana (SAM) use, which results in a greater number and severity of consequences than when either alcohol or marijuana is used alone. Given ongoing legal changes regarding recreational marijuana use, it is likely that SAM use will continue to increase. Most research to date on SAM use has been cross-sectional, retrospective, and/or focused on a single use occasion, and the bulk of research occurred prior to the recent changes in marijuana laws. Further, little is known about what distinguishes frequent SAM users from infrequent SAM users and non-SAM alcohol and marijuana users, and even less is known about specific occasions of SAM use. Therefore, it is imperative to understand more about the epidemiology of this phenomenon and the potentially unique motivational and contextual factors influencing SAM use, as well as subjective effects and use-related consequences. The specific aims of this study are to: 1) obtain contemporary data comparing occasions of SAM use to occasions of alcohol and marijuana use alone, in terms of prevalence, frequency, patterning, mode/type of use, and use- related consequences; and 2) test a developmental, interactive model examining within- and between- individual proximal and distal predictors of SAM use and examine the moderating effects of motivations and contexts on patterns of SAM use, occasions of SAM use, and subsequent negative consequences. We will compare occasions of SAM use to separate occasions of alcohol and marijuana use (within-individual comparison) and compare frequent SAM users to infrequent SAM users and non-SAM alcohol and marijuana users (between-individual comparison), as well as between-individual differences in within-individual associations. This project will examine present-day SAM use among 1,440 college students (ages 18-24) at three state universities, each with very different state laws regarding marijuana use, using a two-phase approach: a larger survey assessment (Phase 1) coupled with a fine-grained daily survey design (Phase 2). The longer survey will be administered prior to each 4-week daily assessment period to 480 students per campus. Daily web-based surveys will also be administered to a subsample of 120 students per campus for 4 weeks during the fall and again during the spring. A secondary aim of this study is to determine if the motivational and contextual influences on SAM use generalize across sites varying in marijuana legal status. Multilevel models will be used to determine predictors and outcomes as a function of whether use is simultaneous or separate. This innovative, multi-site study will provide a theoretical and empirical basis for designing preventive interventions to reduce SAM use and related consequences by determining motivational and contextual targets for interventions as well as actual and perceived norms regarding SAM use. The results of the proposed research will provide critical and timely information to meet the needs of college administrators and counselors and also inform the marijuana legalization debate.